Method of predicting quench cracking in components formed by high deformation processes

ABSTRACT

A process for heat treating a component formed of an alloy. The process includes manipulating uniaxial strain test data of the alloy using a triaxiality factor to determine an equivalent multiaxial stress state. Conditions are then applied to the multiaxial stress state to identify a cooling path for the component. The cooling path includes boundaries for heat treatment temperatures and cooling rates that do not exceed predetermined stresses or strains and/or avoid predetermined residual stress patterns in the alloy. The component is then heated to a heat treatment temperature and quenched according to the cooling path identified in the applying step.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/581,354, filed Dec. 29, 2011, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to components that are produced by forging processes and then heat treated to obtain desirable microstructures. More particularly, this invention is directed to a method for avoiding quench cracking in highly alloyed nickel-base alloys, for example, of the type used to form rotating components of a turbomachine, whose processing includes heat treatment parameters capable of obtaining optimal microstructures.

Components within the combustor and turbine sections of a gas turbine engine are often formed of superalloy materials in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases produced in the combustor. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor components, including blades, spools, disks (wheels) and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, rotating hardware such as turbine disks and compressor spools and disks are typically formed of alloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a controlled grain structure and desirable mechanical properties.

Notable examples of alloys used in these applications include gamma prime (γ′) precipitation-strengthened nickel-base superalloys containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal elements that combine with nickel to form the gamma (γ) matrix, and contain aluminum, titanium, tantalum, niobium, and/or vanadium as principal elements that combine with nickel to form the gamma prime precipitate strengthening phase, principally Ni₃(Al,Ti). Particular examples of gamma prime nickel-base superalloys include Rene 88DT (R88DT; U.S. Pat. No. 4,957,567) and Rene 104 (R104; U.S. Pat. No. 6,521,175), as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. R88DT has a composition of, by weight, about 15.0-17.0% chromium, about 12.0-14.0% cobalt, about 3.5-4.5% molybdenum, about 3.5-4.5% tungsten, about 1.5-2.5% aluminum, about 3.2-4.2% titanium, about 0.5.0-1.0% niobium, about 0.010-0.060% carbon, about 0.010-0.060% zirconium, about 0.010-0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium, the balance nickel and incidental impurities. R104 has a nominal composition of, by weight, about 16.0-22.4% cobalt, about 6.6-14.3% chromium, about 2.6-4.8% aluminum, about 2.4-4.6% titanium, about 1.4-3.5% tantalum, about 0.9-3.0% niobium, about 1.9-4.0% tungsten, about 1.9-3.9% molybdenum, about 0.0-2.5% rhenium, about 0.02-0.10% carbon, about 0.02-0.10% boron, about 0.03-0.10% zirconium, the balance nickel and incidental impurities. Disks and other critical gas turbine engine components are often forged from billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Forging is typically performed on billets having a fine-grained microstructure that promotes formability, after which a heat treatment is often performed to cause uniform grain growth (coarsening) to optimize properties. This heat treatment is performed at a supersolvus temperature, in other words, above the solvus temperature at which the gamma prime precipitates of the alloy enter into solid solution. The forging is then cooled according to a specific cooling process to obtain desired precipitate strengthened microstructures within the disk.

A turbine disk 10 of a type known in the art is represented in FIG. 1. The disk 10 generally includes an outer rim 12, a central hub or bore 14, and a web 16 between the rim 12 and bore 14. The rim 12 is configured for the attachment of turbine blades (not shown) in accordance with known practice. A bore hole 18 in the form of a through-hole is centrally located in the bore 14 for mounting the disk 10 on a shaft, and therefore the axis of the bore hole 18 coincides with the axis of rotation of the disk 10. The disk 10 is a unitary forging and representative of turbine disks used in aircraft engines, including but not limited to high-bypass gas turbine engines such as the GE90® and GEnx® commercial engines manufactured by the General Electric Company.

The bore 14 and web 16 of the turbine disk 10 (as well as those of compressor spools and disks) generally have lower operating temperatures than the rim 12. It is therefore permissible and often desirable that the bore 14 have different properties than the rim 12. Depending on the particular alloy or alloys used, optimal microstructures for the rim 12, bore 14 and web 16 can also differ. For example, a relatively fine grain size is often optimal for the bore 14 and web 16 to promote tensile strength, burst strength, and resistance to low cycle fatigue (LCF), while a coarser grain size is often optimal in the rim 12 to promote creep, stress-rupture, and crack growth resistance, for example, low dwell (hold-time) fatigue crack growth rates (DFCGR) at high temperatures. To satisfy these competing requirements, disks have been proposed that are formed of multiple alloys and/or have different microstructures within the rim and bore. For example, U.S. Pat. Nos. 4,820,358, 5,527,020, 5,527,402 and 6,478,896 disclose dual heat treatment techniques capable of producing single-piece, constant-composition disks having coarser grains within the rim and finer grains with the bore as a result of performing heat treatments at different temperatures on the rim and bore, thereby obtaining the different grain structures and resulting different properties.

Forging conditions, high temperature heat treatments, quench rates, and advanced Ni-base compositions containing high gamma prime content that, in combination, are capable of producing turbine disks with desirable geometries and high temperature properties also result in the disks being highly susceptible to quench cracking, in other words, cracking during the quench step due to the use of a high cooling rate. Though the susceptibility to quench cracking can be reduced by limiting the cooling (quench) rate after a solution heat treatment, doing so limits the processing flexibility necessary in thick sections of a forging to obtain desirable microstructures and properties. The susceptibility to quench cracking has also imposed significant limitations on the chemistries and solvus temperatures of disk alloys along with forging geometry, which limits the ability to maximize mechanical properties.

In view of the above, it can be appreciated that it would be desirable if a method were available that was capable of maximizing cooling rates of thermal treatment processes used to produce turbine disks and/or other components susceptible to quench cracking, thereby expanding the flexibility of these processes.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides processes for heat treating components to have regions with desirable microstructures, and components produced by such processes. Nonlimiting examples include rotating components of turbomachines, including turbine disks of gas turbine engines.

According to a first aspect of the invention, a process is provided for heat treating a component formed of an alloy. The process includes manipulating uniaxial strain test data of the alloy using a triaxiality factor to determine an equivalent multiaxial stress state. Conditions are then applied to the multiaxial stress state to identify a cooling path for the component. The cooling path includes boundaries for heat treatment temperatures and cooling rates that do not exceed predetermined stresses or strains and/or avoid predetermined residual stress patterns in the alloy. The component is then heated to a heat treatment temperature and quenched according to the cooling path identified in the applying step.

According to a second aspect of the invention, a process is provided for heat treating a turbine disk of a gas turbine engine. The process includes manipulating uniaxial strain test data on a precipitation-strengthened alloy using a triaxiality factor to determine an equivalent multiaxial stress state. The turbine disk is formed of the precipitation-strengthened alloy. Conditions are then applied to the multiaxial stress state to identify a cooling path for the turbine disk. The cooling path comprises boundaries for heat treatment temperatures and cooling rates that do not exceed predetermined stresses or strains and/or avoid predetermined residual stress patterns in the precipitation-strengthened alloy. The turbine disk is then heated to a heat treatment temperature and quenched according to the cooling path identified in the applying step.

A technical effect of the invention is the ability to produce a component from a Ni-base composition that has a high gamma prime content and under forging conditions, high temperature heat treatments and quench rates that, in combination, are capable of achieving desirable undistorted geometries, microstructures, and high temperature properties.

Other aspects and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a turbine disk of a type used in gas turbine engines.

FIG. 2 outlines steps performed within a method according to an embodiment of the present invention.

FIG. 3 contains two graphs that relate uniaxial fracture strain data, temperature, and location (relative to the surface of a forging) to the triaxiality factor employed by the present invention to predict quench cracking.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to rotating hardware of the type used in turbomachines, and particularly turbine and compressor disks and compressor spools of high-bypass gas turbine engines. However, it should be understood that the teachings and benefits of the invention are not limited to such hardware, and instead can be adapted and applied to hardware used in a wide range of applications. For convenience, the invention will be described in particular reference to the turbine disk 10 represented FIG. 1, though it should be understood that the teachings and benefits of the invention are not limited to this particular disk 10.

In certain embodiments of the invention, the rim 12, bore 14 and web 16 are all formed of the same alloy. Preferred alloys are strengthened with a precipitation phase that can be solutioned during processing of the alloys. In the context of forming the turbine disk 10, preferred alloys are gamma prime precipitation-strengthened nickel-base alloys, and particular alloys can be chosen on the basis of the operating conditions to which the final product will be subjected. Nonlimiting examples of suitable materials include the aforementioned gamma prime nickel-base superalloys R88DT and R104, as well as certain nickel-base superalloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®. Of particular interest to the invention are Ni-base compositions that have high gamma prime contents, for example, about 30 volume percent and above, and more preferably, about 42 volume percent and above, and most preferably about 49 volume percent and above with a solvus temperature higher than about 2100° F. (about 1150° C.), which include R88DT and R104.

The invention involves forging processes, heat treatments, and quench rates capable of producing different grain sizes and optimal precipitate size and size distribution in different locations of the final product, with grain sizes in specific locations being tailored for the service conditions at those locations. Using the disk 10 of FIG. 1 as an example, a preform can be initially forged at relatively high strain rates to generate a fine-grained structure within at least a portion of the resulting forged profile, after which the profile undergoes a heat treatment and quench to cause or promote the generation of different grain sizes in different locations of the disk 10. The heat treatment can be a supersolvus heat treatment to dissolve the gamma prime precipitates and cause recrystallization and grain growth that occurs as a result of plastic strain retained within the forged profile from the preceding forging process, after which the quench is performed following a suitable cooling path to achieve a desired grain size and form a desired size and size distribution of gamma prime precipitates. As used herein, the term cooling path refers to a process involving any number of heat treatment temperatures, hold times, and quench rates performed in a specific order so as to obtain a desired microstructure in a component.

Forging preforms can be produced by a variety of known processes, including billets produced by powder metallurgy (P/M), conventional cast and wrought processing, and spraycast or nucleated casting forming techniques. Preforms will typically have a fine grain size, for example, an average grain size of about ASTM 10 or finer, to promote forgeability. During forging, the preform can be forged at a temperature, strain level and strain rates that will promote desired grain sizes, for example, finer grains within the portion of the profile that will eventually define the bore 14 of the disk 10. The strain level in the preform is preferably sufficiently high to cause recrystallization and grain growth during a subsequent heat treatment, which is preferably carried out to obtain a microstructure having desirable characteristics. As a nonlimiting example, if the alloy is a precipitation-strengthened alloy such as R88DT or R104, the final heat treatment is preferably performed at a supersolvus (solution) heat treatment temperature, in other words, at a temperature higher than the solvus temperature of the alloy. During the supersolvus heat treatment, gamma prime precipitates within the profile dissolve, which permits recrystallization and grain growth to occur. Thereafter, the profile is cooled according a predetermined cooling path. For example, the profile may be initially cooled slowly to reduce plastic strain built up in the profile to avoid the quench cracking. This brief period of slow cooling will likely have little impact on the desired precipitation, due to the undercooling of the precipitation kinetics. Once precipitation commences, the desired microstructure and quench cracking limits are balanced. In addition, once at this lower temperature (later stage of quench), a more rapid quench may be used. As can be seen, the cooling path (dependent on time and temperature) likely includes multiple cooling rates and times.

As previously noted, the quench rate is typically limited to avoid quench cracking. Advanced P/M disks formed of Ni-base alloys that are highly alloyed to contain a high gamma prime content are known to be particularly susceptible to quench cracking. Typical measures taken to avoid quench cracking have posed significant limits on the chemistry, gamma prime content, and properties of Ni-base alloys. Prior attempts to predict quench cracking based on the use of uniaxial strain-to-crack or stress-to-crack criteria have not faithfully accounted for the complicated multi-dimensional strain or stress states that exist within the geometries of disks and other components produced by forging and other high-deformation processes.

For the purpose of modeling and predicting the potential for quench cracking, the present invention identifies the triaxiality effect as a critical addition to the quench crack analysis in conjunction with previously developed cooling rate sensitive strain-to-crack criteria. Quench cracking criteria utilizing this principle have the potential for allowing highly-alloyed Ni-base alloys to be quenched without cracking, while also achieving refined microstructures capable of promoting desirable properties, such as those required for disks and other rotating hardware of turbomachines.

The present invention makes use of a method of modeling a realistic strain-to-crack value caused by the triaxiality of the stress and strain state in a forging, in conjunction with uniaxial tests and finite element modeling (FEM) of the forging. The model corrects for the reduction of ductility by triaxiality that would result from a purely uniaxial analysis. With more accurate quench crack criteria obtainable with the invention, heat treatment process windows and equipment can be designed to achieve specific and more optimal microstructures and properties within certain locations of a forging.

As represented in FIG. 2, the modeling method involves first obtaining data with uniaxial strain tests relating to the material of the desired component. For example, tensile tests may be performed on a Ni-based alloy of interest having a similar microstructure to the desired component, for example, fine grains near the bore 14 of the disk 10. The tensile tests are performed at various temperatures to determine the uniaxial fracture strain of the alloy at the tested temperatures. The results of the tensile tests may be compiled in a table of values listing the maximum uniaxial strain (dependent on temperature and cooling rate) achieved with the alloy without necking or voiding, which can then be used to predict whether or not quench cracking might occur as a result of a particular heat treatment condition.

In the second step of FIG. 2, the table of values resulting from the uniaxial strain tests is modified by inserting the values into formulas (Manjoine formulation) which account for triaxiality, which is location and temperature dependent. The formulas are as follows:

$F_{T} = \frac{\sqrt{2}\left( {\sigma_{1} + \sigma_{2} + \sigma_{3}} \right)}{\sqrt{\left( {\sigma_{1} - \sigma_{2}} \right)^{2} + \left( {\sigma_{2} - \sigma_{3}} \right)^{2} + \left( {\sigma_{3} - \sigma_{1}} \right)^{2}}}$ $ɛ_{f} = {{Min}\left\lbrack {\frac{ɛ_{u}}{F_{T}};{ɛ_{u}\left( 2^{1 - F_{T}} \right)}} \right\rbrack}$

where F_(T) is the triaxiality factor, σ_(i) is the principle stresses (i=1, 2, 3), ε_(u) is the fracture strain for uniaxial loading, and ε_(f) is the equivalent strain at fracture for multiaxial loading. It is well known that ductile fracture is strongly dependent on triaxiality, which is the ratio of hydrostatic and equivalent stresses. The resulting triaxiality-based strain-to-crack values (ε_(f)) are believed to address the ductility reduction in the material, or the decrease in the strain level at failure, due to multiaxial loading effects. Therefore, these values are believed to more accurately predict quench cracking within the alloy under given heat treatment conditions, including temperatures and cooling rates. The uniaxial strain-to-crack test data (ε_(u)) provide fracture strain for an equivalent multiaxial stress state (ε_(f)). With the invention, ductile fracture strain under any multi-stress state can be related to a measurable uniaxial fracture strain for specific heat treatment temperatures and cooling rates. These resulting values represent specific stresses or strains and/or specific residual stress patterns that should be avoided in the alloy in order to avoid quench cracking.

FIG. 3 contains two graphs that relate uniaxial fracture strain data, temperature, and location (relative to the surface of a forging) to the triaxiality factor (F_(T)) employed by the present invention to predict quench cracking. Graph A represents the relationship between a ratio of actual fracture strain, uniaxial fracture strain, and triaxiality. This relationship was verified by U notch tests using nickel-base superalloy specimens that were evaluated during investigations leading to the present invention. In graph B, point one (P1) is a position level with a surface of the specimen and the other points (P2-P6) are positioned incrementally 0.05 inches (about 1.27 mm) away from the surface and P1.

For the third step of FIG. 2, conditions are applied to the resulting triaxiality-based strain-to-crack values computed with the two equations above using the triaxiality factor to identify boundaries for heat treatment parameters that avoid strain values that would cause quench cracking. This may be accomplished by imputing the data into a FEM-based heat treatment model to compute the dynamic triaxiality under quench conditions, by which the locus of fracture strain close to the surface under the quench conditions are mapped out. Any total effective strain in the vicinity of the surface that exceeds the triaxiality corrected fracture strain computed by FEM is predicted to result in a quench crack. This information is used to identify boundaries of the potential heat treatment (quench cooling rates) parameters that avoid strain values that would cause quench cracking. The identified boundaries for the heat treatment parameters may be further validated by performing quenching tests on subscale coupons and full scale production parts with various quench media.

By utilizing the above described prediction model, the disk heat treatments (cooling path) can be designed to achieve the highest cooling rate possible for improved properties while avoiding quench cracking, for example, by controlling the heat treat delay transfer time and the localized ramped gas cooling. More particularly, and corresponding to the fourth and fifth steps identified in FIG. 2, a preform of the alloy used as the basis for the preceding steps is then forged and heat treated at or approaching the boundaries for the strain rate values and heat treatment parameters that avoid quench cracking that were identified in the preceding steps. The strain rates and heat treatment conditions can be selected to optimize strain rates or heat treatment conditions (including temperatures and cooling rates), depending on the particular microstructure (for example, grain sizes) and properties desired for the forging following heat treatment. With these additional steps, the method of this invention has been experimentally validated in production trials on gas turbine disks.

While the invention has been described in terms of a specific embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the component could differ from that shown, and materials and testing methods other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims. 

1. A process of heat treating a component formed of an alloy, the process comprising: manipulating uniaxial strain test data of the alloy using a triaxiality factor to determine an equivalent multiaxial stress state; applying conditions to the multiaxial stress state to identify a cooling path for the component, wherein the cooling path comprises boundaries for heat treatment temperatures and cooling rates that do not exceed predetermined stresses or strains and/or avoid predetermined residual stress patterns in the alloy; and then heating the component to a heat treatment temperature and quenching the component according to the cooling path identified in the applying step.
 2. The process of to claim 1, wherein the alloy is a precipitation-strengthened alloy.
 3. The process of claim 2, wherein the precipitation-strengthened alloy is a nickel-base alloy comprising gamma prime precipitates.
 4. The process of claim 2, further comprising performing uniaxial strain tests on the alloy to obtain the uniaxial strain test data prior to the manipulating step.
 5. The process of to claim 2, wherein the temperature of the heating step is a supersolvus temperature of the precipitation-strengthened alloy.
 6. The process according to claim 2, wherein the precipitation-strengthened alloy has a gamma prime volume fraction of about 49% and above with a solvus temperature higher than about 1150° C.
 7. The process of to claim 1, wherein portions of the component have different average grain sizes following the heating step.
 8. The process of claim 1, further comprising calculating the triaxiality factor for the precipitation-strengthened alloy by inputting the uniaxial strain test data into the equation: $F_{T} = \frac{\sqrt{2}\left( {\sigma_{1} + \sigma_{2} + \sigma_{3}} \right)}{\sqrt{\left( {\sigma_{1} - \sigma_{2}} \right)^{2} + \left( {\sigma_{2} - \sigma_{3}} \right)^{2} + \left( {\sigma_{3} - \sigma_{1}} \right)^{2}}}$ prior to the manipulating step, wherein F_(T) is the triaxiality factor and σ₁, σ₂, and σ₃ are principle stresses.
 9. The process of claim 1, wherein the manipulating step comprises calculating equivalent strain at fracture for multi-axial loading by inputting the uniaxial strain test data and the triaxiality factor into the equation: $ɛ_{f} = {{Min}\left\lbrack {\frac{ɛ_{u}}{F_{T}};{ɛ_{u}\left( 2^{1 - F_{T}} \right)}} \right\rbrack}$ wherein F_(T) is the triaxiality factor, ε_(u) is the fracture strain for uniaxial loading, and ε_(f) is the equivalent strain at fracture for multi-axial loading.
 10. The process according to claim 1, wherein the component is a rotating component of a gas turbine engine.
 11. The process according to claim 10, wherein the rotating component is a turbine disk.
 12. A process of heat treating a turbine disk of a gas turbine engine, the process comprising: manipulating uniaxial strain test data on a precipitation-strengthened alloy using a triaxiality factor to determine an equivalent multiaxial stress state, wherein the turbine disk is formed of the precipitation-strengthened alloy; applying conditions to the multiaxial stress state to identify a cooling path for the turbine disk, wherein the cooling path comprises boundaries for heat treatment temperatures and cooling rates that do not exceed predetermined stresses or strains and/or avoid predetermined residual stress patterns in the precipitation-strengthened alloy; and then heating the turbine disk to a heat treatment temperature and quenching the turbine disk according to the cooling path identified in the applying step.
 13. The process of to claim 12, wherein the precipitation-strengthened alloy is a nickel-base alloy comprising gamma prime precipitates.
 14. The process of to claim 12, wherein the precipitation-strengthened alloy has a gamma prime volume fraction of about 49% and above with a solvus temperature higher than about 1150° C.
 15. The process of claim 12, further comprising performing uniaxial strain tests on the alloy to obtain the uniaxial strain test data prior to the manipulating step.
 16. The process of to claim 12, wherein the heat treatment temperature is a supersolvus temperature of the precipitation-strengthened alloy.
 17. The process of to claim 12, wherein portions of the turbine disk have different average grain sizes following the heating step.
 18. The process of claim 12, further comprising calculating the triaxiality factor for the precipitation-strengthened alloy by inputting the uniaxial strain test data into the equation: $F_{T} = \frac{\sqrt{2}\left( {\sigma_{1} + \sigma_{2} + \sigma_{3}} \right)}{\sqrt{\left( {\sigma_{1} - \sigma_{2}} \right)^{2} + \left( {\sigma_{2} - \sigma_{3}} \right)^{2} + \left( {\sigma_{3} - \sigma_{1}} \right)^{2}}}$ prior to the manipulating step, wherein F_(T) is the triaxiality factor and σ₁, σ₂, and σ₃ are principle stresses.
 19. The process of claim 12, wherein the manipulating step comprises calculating triaxiality-based strain-to-crack values by inputting the uniaxial strain test data and the triaxiality factor into the equation: $ɛ_{f} = {{Min}\left\lbrack {\frac{ɛ_{u}}{F_{T}};{ɛ_{u}\left( 2^{1 - F_{T}} \right)}} \right\rbrack}$ wherein F_(T) is the triaxiality factor, ε_(u) is the fracture strain for uniaxial loading, and ε_(f) is the equivalent strain at fracture for multi-axial loading.
 20. The process of claim 12, further comprising forging a preform formed of the precipitation-strengthened alloy to produce the turbine disk. 